The present invention relates to computer-aided modeling of geological channels.
Methods are known, notably in oil exploration, for determining the position of oil-bearing reservoirs from the results of geophysical measurements performed at the surface or within drillings. One of the most useful prospecting methods employed in this area is probably seismology which makes it possible to visualize deep geological structures by analyzing seismic wave echoes. These seismic waves can be of natural origin (for example an earthquake) or of artificial origin, created for example by means of explosives, a vibrating truck, etc.). The seismic reflection is distinguished from seismic refraction. The seismic reflection is employed to determine interfaces between geological strata.
Among known geological structures, so-called channels (or paleochannels) are of particular interest in oil exploration. A channel is for example defined as a water conduit originating from the continent, carrying sand or sediment, which follows a variable winding course along a marine slope. When a marine flow path gets formed, this creates a channel which gradually gets filled up by sediments carried along by the fluid. Such sediments are called turbidites. Then, when flow starts again after a prolonged break or when there is some abrupt variation in flow rate for example, a new channel gets formed which is shifted to a greater or lesser degree with respect to the former channel. This is called channel migration. A succession of such sequences leads to the formation of a channel complex or fairway.
More precisely, the channel complex represents a conduit for transit of sediment, running from the continental shelf towards deep water. It exhibits a character which is frequently erosive and develops on the continental slope. It is made up of an amalgam of individual turbidite channels representing different sediment transport phases and events.
Channel complexes (or channel fairways) have a characteristic geometric shape with stacking of the channels making them up. Further, the sediment filling of the channel complexes is different from that of its environment. The channel complex thus has a relatively precise envelope, and it is possible to locate its margin. The margins of the channel complex can then be determined from an isopach map produced between two seismic horizons defining the base of the channel complex or fairway and the top of the complex filling.
Channel complexes and more precisely the channels themselves are potential oil bearing reservoirs; they possess the major characteristics such as high porosity or high permeability. Such channels are thus likely to contain oil that it is desired to extract; it is consequently useful to model their formation. Their sinuous geometry nevertheless makes them difficult to model.
In the modeling of subsoil properties, one can generally distinguish two families of methods: pixel-based methods where the property is calculated at each node (pixel) of a discrete model (a structured grid), and object-based methods—also called Boolean methods—in which geological objects (channels, lobes, dunes, etc.) are constructed and then employed to estimate values of the property in the reservoir model.
Pixel-based methods are not particularly suitable for channel modeling as they do not make it possible to reproduce the curvilinear and continuous shape of the deposit, for example fluvial depositions.
A very large number of object-based methods exist including two families between which a distinction is made: deterministic methods and probabilistic methods. The results that are obtained have many similarities, even though the methodological approaches used are completely different.
In deterministic methods, one sets out to reproduce the evolution of the channel in time and in space. These methods are based on geological and sedimentalogical concepts along with data from seismic visualization. Deterministic methods give very realistic results but can only be performed in the case where the objects are observable on seismic datasets.
In the case of probabilistic methods, each geological object is defined as a more or less complex geometrical shape. Typically, the channels are defined starting from a sinuous curve the distance from the channel axis of which is modeled using a stochastic process. This simulated curve is employed as a channel curve. Channel cross section is then calculated at every point on this curve, with generally variable thickness and width. The results that are obtained are much less realistic than with the deterministic methods. In order to overcome this problem, there have been proposals to deform the curve obtained using its geometry and a law which complies with the erosion at the convex edge of the channel. The results obtained are then more realistic and sinuous than the previous results.
Boolean methods of the probabilistic type do notably suffer from the defect of not allowing account to be taken of major sinuosities, thereby giving rise to poorly realistic geological models.
European patent application EP-A-1,584,951 provides improvements to the state of the art by providing prior parameterization of a reference geological horizon H, with a view to stochastically generating the associated paleo-channels or paleo geological structures in the region of this geological reference horizon.
Nevertheless, the known channel modeling methods do not make it possible to obtain simply a “realistic” continuity between channels, notably in regions of lateral migration. This results in the trajectories of development drillings made on the basis of such models failing to be optimal.